1. Field of the Invention
This invention relates to a method for producing vinylalkoxysilanes at high purity. In particular, the present invention is directed to an efficient method for producing vinylalkoxysilanes by direct hydrosilation of an alkyne with alkoxysilane.
2. Description of Related Art
Vinyl silyl esters, also known as vinyl silyl ethers, hereinafter referred to as vinylalkoxysilanes, are commercially important compositions of matter. These materials are used as coupling agents in thermoplastic composites, in fiberglass primers, and in cable insulators. Vinylalkoxysilanes also are used as co-monomers in water-curable (cross-linkable) polyolefin-containing plastics. In this latter application, in particular, it is important that a high source of the vinylalkoxysilane be available, the presence of even small amounts of such impurities as tetraalkoxysilanes and alkylalkoxysilanes in the vinylalkoxysilane is unwanted. These impurities are particularly troublesome because the closeness of their boiling points to that of the desired vinylalkoxysilane makes their removal therefrom via distillation practically impossible.
Diverse methods of manufacturing vinylalkoxysilanes and related compounds are known, but none has been completely satisfactory for producing a high purity product. In one approach, a vinylchlorosilane is esterified with an alcohol in accordance with the following reaction: EQU RCH.dbd.CHSiCl.sub.3 +3R'OH.fwdarw.RCH.dbd.CHSi(OR').sub.3 +3HCl1a
wherein R is an alkyl group or hydrogen, and R' generally is an alkyl or an aryl group. Removal of by-product hydrogen chloride is time consuming and by its nature only partially successful, thus imposing practical purity limits on the product. Furthermore, side reactions encountered during the preparation of the vinylchlorosilane degrades the actual yield of vinylalkoxysilane from the alkyne.
The vinylchlorosilane initially is prepared by hydrosilation of an alkyne with a trichlorosilane, as follows: EQU RC.tbd.CH+HSiCl.sub.3 .fwdarw.RCH.dbd.CHSiCl.sub.3 1b
with R as defined above. Unfortunately, the following side reaction also takes place and reduces the theoretical yield of the desired vinylalkoxysilane from the alkyne: EQU RCH.dbd.CHSiCl.sub.3 +HSiCl.sub.3 .fwdarw.Cl.sub.3 SiCHRCH.sub.2 SiCl.sub.3 1c
In an alternative approach, the vinylalkoxysilane can be prepared by first reacting an alkene with an aminosilane in the presence of a catalyst according to the following reaction: ##STR1##
The vinylaminosilane thus produced then is alkoxylated with an alcohol according to the following reaction: EQU RCH.dbd.CHSi(NMe.sub.2).sub.3 +3R'OH.fwdarw.RCH.dbd.CHSi(OR').sub.3 +3HNMe.sub.2 2b
As with the prior method, side reactions and the presence of by-products, particularly dimethylamine, reduce the theoretical yield and purity of the desired vinylalkoxysilane product.
The prior art also has disclosed the direct hydrosilation of an alkyne using an alkoxysilane over a variety of catalysts, in accordance with the following formula: EQU RC.tbd.CH+HSi(OR').sub.3 .fwdarw.RCH.dbd.CHSi(OR').sub.3 3a
This approach is potentially superior to the previously described routes because of its simplicity as a one-step reaction and because formation of the desired vinylalkoxysilane, in theory, is not necessarily accompanied by the formation of certain by-products such as dimethylamine or hydrogen chloride.
U.S. Pat. No. 2,637,738 describes the reaction of triethoxysilane with acetylene in the presence of a catalyst comprising platinum supported on finely divided charcoal to form vinyltrialkoxysilane. The reaction was conducted at a temperature of about 130.degree. C. and at a gas pressure of from about 2 atmospheres (Table III), where no vinylsilane was formed, up to about 20 atmospheres (Example 2), where the vinylsilane was the major product, by charging acetylene to an agitated reactor containing the triethoxysilane and the platinum catalyst. Unfortunately, the reported yield of the desired vinyltrialkoxysilane is rather low, being accompanied in all reported cases by a substantial quantity of by-product bis(alkoxysilyl)alkane, i.e., 1,2-bis(triethoxysilyl)ethane, produced as a consequence of the following reaction: EQU RCH.dbd.CHSi(OR').sub.3 +HSi(OR').sub.3 .fwdarw.(R'O).sub.3 SiC(R)HCH.sub.2 Si(OR').sub.3 3b
Still other by-products include the related alkylalkoxysilanes and the tetraalkoxysilane whose formation by the following reactions normally is favored by elevated reaction temperatures: EQU 2HSi(OR).sub.3 .fwdarw.Si(OR).sub.4 +H.sub.2 Si(OR).sub.2 3c EQU CH.sub.2 .dbd.CHSi(OR).sub.3 +H.sub.2 .fwdarw.CH.sub.3 CH.sub.2 Si(OR).sub.3 3d EQU CH.tbd.CH+H.sub.2 +HSi(OR).sub.3 .fwdarw.CH.sub.3 CH.sub.2 Si(OR).sub.3 3e
Hydrogen for these reactions can be formed during the preparation of the desired product by dehydrogenation of the platinum/vinyl complex which is formed during the reaction sequence in which the bis(alkoxysilyl)alkane compound is formed. Hydrogen also may possibly be formed by the following condensation reaction: EQU 2HSi(OR).sub.3 .fwdarw.(RO).sub.3 SiSi(OR).sub.3 +H.sub.2 3f
A method disclosed in U.S. Pat. No. 2,823,218 utilizes chloroplatinic acid (H.sub.2 PtCl.sub.6.6H.sub.2 O) to catalyze reactions involving a silicon-hydrogen bond and an unsaturated carbon-carbon bond. The method is said to increase the yield of the desired product, decrease the yield of by-products, and lower the necessary reaction temperature. Example 21 describes adding acetylene to a reactor containing triethoxysilane and an isopropanol solution of chloroplatinic acid to produce vinyltrialkoxysilane. Yield and purity information is not reported.
Japanese Kokai 57/4995 (1982), in Comparison Example 13, discloses adding acetylene to a reactor containing 80 mmol of trimethoxysilane and chloroplatinic acid catalyst solution dissolved in 70 ml of xylene as a solvent. The hydrosilation reaction was conducted at a reaction temperature of 60.degree. C. and at normal pressure. A major product formed was 1,2-bis(trialkoxysilyl)ethane.
Reaction of a tri-t-alkoxysilane with an alkyne in the presence of a platinum hydrosilation catalyst at a temperature greater than about 150.degree. C. to produce a vinyltri-t-alkoxysilane is disclosed in U.S. Pat. No. 4,579,965. The patent discloses that production of undesired bis(tri-t-alkoxysilyl)ethane is very low. Suggested platinum hydrosilation catalysts include platinum metal (alone or on a support), chloroplatinic acid, and platinum(II) 2,4-pentanedionate.
Notably, this patent discloses that when a primary or secondary alkoxysilane (e.g., trimethoxysilane, triethoxysilane, or triisopropoxysilane), instead of the tertiary alkoxysilane, is reacted with acetylene, formation of the undesired bis(silylalkoxy)alkane derivative predominates over the desired reaction yielding the vinylalkoxysilane. In particular, Comparative Examples 2 through 4 show that the bis-silyl derivative predominates when acetylene is added to a mixture of a primary or secondary trialkoxysilane and a chloroplatinic acid catalyst. Although relative selectivity improved when PtCl.sub.2 (PPh.sub.3).sub.2 was utilized in place of chloroplatinic acid catalyst, desired product yield remained at only 80 percent for primary alkoxysilanes. Furthermore, in each such case, the yield of tetraalkoxysilane and alkylalkoxysilanes by-products was at least 2 wt. percent. As noted above, these unwanted by-products are exceedingly difficult to remove from the desired vinylsilane product.
Some approaches for increasing the yield of the desired vinylsilane produced via direct hydrosilation of an alkyne using a trialkoxysilane are not commercially practicable. One possible expedient involves significantly increasing the pressure of the alkyne, (RC.tbd.CH), such as acetylene, to as high as 20 atmospheres. Unfortunately, an acetylene pressure higher than about 1 to 4 atmospheres (gauge) creates a significant safety hazard.
U.S. Pat. No. 3,793,358 discloses a method for manufacturing alkenylhaloalkylsilanes (vinylhaloalkylsilanes) from a halosilane and an alkyne. Hydrosilation of an alkyne with a trialkoxysilane is not disclosed or suggested. In accordance with the disclosed method, a mixture of the alkyne and a silane having one or two silicon-hydrogen bonds, with the remaining silicon valence bonds occupied by a halogen or an inert monovalent organic radical, such as an alkyl, aryl, or cycloalkyl radical, is added to a reactor containing an addition catalyst and a disilylethane (bis-silylethane) solvent or diluent. The disilylethane used is preferably the same one which is formed as a by-product of the hydrosilation reaction. A reaction temperature between 120.degree. and 220.degree. C. and a reaction pressure between 0.1 and 5.0 atmospheres are utilized. Suitable addition catalysts include chloroplatinic acid and preferably its reaction products and complexes. This patent does not describe how to minimize or avoid the formation of inseparable by-products, such as the alkylsilanes, when producing vinylalkoxysilanes.
The prior art has failed to define a direct process for producing high-purity vinylalkoxysilanes in high yield from an alkoxysilane.